Conveying distance across the multiple planes of a landscape can be handled by muting the local color with grays, blues and the color's complement, as well as in the technique of modifying the applied pastel with a tortillion or the edge of a hard pastel.
Once a group of tree branches is drawn, a tortillion can be used to soften the branches as well as create softer limbs with the pointed edge of the tortillion (after it has accumulated some of the applied pastel.
Another technique of creating distance is to softly scumble the complementary color on the applied pastel. In the example below, I take an orange pastel and lightly draw it across the deep blue mass of trees. The local "bold" blue color will recede in the distance and not look as vibrant as if it were up close.
Similarly, a blue pastel helps soften the brilliance of the distant color to "gray it out." In this example, the bright orange tree is blurred and made more hazy with the blue complement.
The tip of the tortillion can also be used to build soft tips of grasses and shrubs by pulling the pastel of the shrubs up to a soft point, as can be seen below in this portion of "Birch Ballet."
A composition can be designed not only by the placement of the objects and their relationships in the landscape, but also by defining a variety of "planes" and conveying them with different values (i.e. shade and sun) as well as distinct color intensities. For example, the foreground of a landscape will show sharp edges of bold local color, while the distant horizon will be softer, less detailed and more gray.
In "Songbird Field" below, note the sharp flowers and foliage in the foreground, and the muted distant grasses and tree line at the horizon.
In "Morning Treats" below, shade and muted distant color is used to enhance the composition and to convey distance.
Hard pastels (actually, the hard "soft" pastels such as the NuPastel brand) and pastel pencils, are effective ways to blend and soften distant edges. Having more tools (and pastel types) handy will help the pastel artist complete a variety of effects necessary to accurately portray a variety of landscapes.